1. Field of the Invention
The present invention relates to a heart stimulating device having a capability to adapt its stimulation energy to the current capture threshold value in a patient's heart.
2. Description of the Prior Art
The invention concerns solutions to problems arising in sensing the activity of the heart when myopotential noise, external electromagnetic noise, or other noise is present.
Particularly, noise may severely affect sensing carried out with a unipolar lead connecting the heart and the stimulating device. For unipolar electrode systems the noise level is typically an order of magnitude higher than the noise level of bipolar electrode systems. Aggravating polarization problems may thus arise when stimulation and sensing employ the same polarity configuration, e.g., unipolar configuration.
Pacemakers may employ a unipolar or bipolar lead, i.e., a cardiac electrode arrangement plus a single or dual insulated wires or conductors connected to the pacemaker's connection terminal, for stimulating and sensing the activity of a patient's heart. A bipolar lead may be operated in bipolar or unipolar mode, but a unipolar lead can be operated only in unipolar mode.
Stimulation and sensing carried out on a common unipolar lead would be generally desirable provided that safety can be maintained. Two advantages of unipolar systems are the greater possibilities of using an already-implanted lead when the pacemaker needs to be replaced, and a lower number of components which may fail.
Some terminology used in this disclosure is explained below.
IEGM
An abbreviation for intracardiac electrogram. IEGM signals are emitted by active cardiac tissue and sensed through electrodes placed on or within the heart.
QRS or QRS complex
The ventricular depolarization as seen on the electrocardiogram or in the IEGM signals.
Intrinsic
Inherent or belonging to the heart itself. An intrinsic beat is a naturally occurring heartbeat.
Evoked response
The electrical activation of the myocardium caused by a pacemaker output pulse. The ability of heart cells to respond to a pacemaker output pulse depends on the extent to which the cells are in a refractory state.
Escape interval, basic interval, or basic escape interval
The period, typically of the order of 1000 milliseconds, between a sensed intrinsic cardiac event or a stimulation pulse output and the next pacemaker output pulse.
Lead
The insulated wire plus electrode(s) and terminal pin used to connect the pulse generator to cardiac tissue. The lead carries the stimulus from the pulse generator to the heart and, in demand modes, relays intrinsic cardiac signals back to the sense amplifier of the pulse generator. A single-chamber pulse generator requires one lead, while a dual-chamber pulse generator usually requires two (one for the atrium, another for the ventricle)
Bipolar lead
A pacing lead with two electrical poles that are external from the pulse generator. The negative pole or cathode is located at the extreme distal tip of the pacing lead, while the positive pole or anode is formed of an annular electrode located at a distance in the range of 10 millimeters from the cathode. The cathode is the electrode through which the stimulating pulse is delivered. Bipolar leads are characterized by relatively small spikes on the paced ECG.
Unipolar lead
A pacing lead with a single electrical pole at the distal tip of the pacing lead (negative pole). The anode (positive pole) is the pulse generator case. The cathode is the electrode through which the stimulating pulse is delivered. Unipolar stimulation or sensing via a unipolar lead is of course bipolar in the sense that, e.g., a conductive casing of the pacemaker constitutes a second pole.
Stimulation, capture, or pacing threshold
The minimum electric output from the pacemaker which consistently elicits a cardiac depolarization and contraction.
Stimulation energy
The energy of the electric output from the pacemaker. The energy is used herein to quantitatively describe the stimulation effectiveness of a stimulation pulse. Alternatively, it could be expressed in terms of voltage, current, width, shape, and/or charge of the pulse.
IEGM signal amplitude
The amplitude can be defined in two different ways: the peak value, i.e., the maximum distance (positive and/or negative) from the signal baseline, or the peak-to-peak value, i.e., the distance between the maximum positive and negative deflections of the IEGM signal. An implantable heart stimulating device usually senses the peak-to-peak value. Thus, a threshold for sensing could refer to either of these definitions.
In U.S. Pat. No. 4,969,460 a pacemaker is described having a spontaneous event and noise detector for sensing inter alia noise. When noise is detected a noise flag is set and capture detection and automatic output regulation is suspended (see e.g. col.24, lines 26-48). Instead pacing takes place with a comparatively large output. Criteria for setting the noise flag are not discussed.
The Pacesetter.RTM. REGENCY.TM. pacemaker with AUTOCAPTURE.TM., described in Pacesetter.RTM. User Manual, ordering no. 63 46 493 E500E, published in 1995, uses a bipolar arrangement connecting the pacemaker to the heart when operating in a mode wherein the stimulation pulse energy is adjusted in relation to the patient's changing capture threshold. In that AUTOCAPTURE mode, only one pole of the lead is used for delivering stimulation pulses, while both poles are used for sensing IEGM signals that are indicative of the heart's stimulation response and natural activity. In other modes of operation the pacemaker may be programmed to deliver stimulation pulses and sense heart activity in either bipolar or unipolar mode. The pacemaker may always completely inhibit a pulse, in order to favor natural heartbeats. In that way a long life of the battery-powered heart stimulating device may be achieved.
The adaptive function is advantageous, but to ensure correct evoked response (ER) detection, bipolar sensing is deemed absolutely necessary. In the described pacemaker, unipolar sensing could endanger correct ER detection, since that sensing is more sensitive to noise, especially myopotential, influence than bipolar sensing, as mentioned above. Depending on the sign and magnitude of the noise, ER sensing could erroneously indicate capture or non-capture, which could lead to an inappropriate reaction by the pacemaker.
Also, when sensing and stimulation are carried out with the same polarity, the sensing can be complicated due to polarization at the electrode(s) caused by the stimulation pulse, however, that is a problem not specifically related to the occurrence of noise.
If sensing and pulse adjustment based thereon could be performed reliably with a unipolar lead instead of a bipolar lead, one would also benefit from the advantages of a less complicated conductive means for transferring both the stimulation and sensing signals.
The general operation of a prior art pacemaker of the type initially described will now be described in greater detail.
Such a pacemaker operates in a sensing mode that incorporates a refractory period. Immediately following a pacemaker output or a sensed intrinsic event, the pacemaker ceases to be responsive to detectable signals for a predetermined period of time. This prevents the pulse generator from detecting the terminal portion of the depolarization signal and, in some circumstances, the repolarization signal which might result in timing errors.
In atrial applications, longer refractory periods should be employed to prevent detection of terminal portions of the QRS complex which, if detected, would reset the pulse generator timing, resulting in a lower pacing rate than the programmed rate.
The refractory period (=total refractory period) consists of a programmable absolute refractory period during which detection of all signals is blocked, and a non-programmable relative refractory (or noise-sampling) period (100 ms) during which detected signals cause a restart of the relative refractory period. Continuous detection of signals at a frequency of 10 Hz or more causes the pulse generator to revert to asynchronous operation at the programmed basic rate as long as noise is present. During periods of noise detection, the pulse generator stimulates at the programmed pulse amplitude and pulse width if the adjustment mode (AUTOCAPTURE.TM.) is "OFF". If the adjustment mode is programmed "ON", the algorithm is interrupted and the output set to 4.5 V/0.49 ms to ensure pacing. High stimulation energy is necessary when the adjustment mode is "ON" because the noise will probably give false ER detections. As soon as the noise is absent, the pulse generator reverts to the normal inhibited mode with the same amplitude and pulse width as before the noise mode was entered, when operating in the adjustment mode. In this pacemaker, the noise is detected by the same circuit, with the same amplitude threshold, as used to sense heartbeats.
The programmable refractory periods, including the relative refractory period of 100 ms, range from 250 ms to 550 ms in steps of 50 ms.
Signals which occur at a frequency of 10 Hz or more are interpreted as noise and will cause the pacemaker to revert to asynchronous operation at the programmed rate while continuing to monitor for the presence of noise. This protects the patient by preventing the pacemaker from being totally inhibited by external interference.
Signals which occur at a frequency below 10 Hz have no effect upon pulse generator timing, unless the signal is detected during the normal sensing (or alert) period following the noise sampling period. Should this occur, pulse generator output either will be inhibited or triggered depending on the operating mode.